The present invention relates to a pick-up arrangement for detecting axial displacement of a pressing plunger.
Arrangements of the above-mentioned general type are known in art. One of such arrangements is disclosed, for example, in the U.S. Pat. No. 3,180,718. This arrangement is formed as a displacement pick-up which has a primary winding and two secondary windings of a differential transformer each embedded in an outer peripheral groove of a stationary inlet pipe produced of an electrically insulating material for pressure air cooling of the plunger. The plunger is mounted on a metal tubular piston rod whose free end surrounds the inlet pipe as an actuating element. By the movement of the piston rod relative to the windings, the differential transformer produces a variable output signal which is proportional to the movement of the plunger and represents a value of both the movement and the insertion depth of the plunger into the parison mold. The output signal is advantageously indicated and/or displayed in writing. As an alternative, it can be used also for controlling the gob size. This known pick-up arrangement is relatively expensive, is not sufficiently resistant mechanically bacause of the non-metallic cooling air inlet pipe, and can be handled only by skilled personnel. A temperature compensation is required when in accordance with the conventional practice the cooling air inlet pipe which carries the windings is acted upon only intermittently with cooling air and the temperature of the windings respectively changes constantly within wide limits. Also, a more than two-wire electric connecting cable is required.
The U.S. Pat. No. 4,369,053 discloses a pick-up arrangement for an I.S. glass forming-machine which has a rod displaceably guided in the cylinder wall and provided with a sensing member arranged in the movement path of the plunger or the plungers. The windings of a differential transformer are arranged in an opening provided in the cylinder wall in alignment with the rod. A core connected with the rod and insertable into the windings serves as an actuating element. Despite the fact that in this arrangement a temperature compensation is no longer necessary, the structural expenses in view of the limited space condition in the cylinder wall are relatively high.
The DE-OS No. 2,923,705 discloses an I.S. glass-forming machine in which the end region of the path of the actuating piston of the plunger or the piston rod of the actuating piston is measured. The covered displacement must be detected with the aid of a pick-up and made visible. This is attained for example by several electrical contacts arranged in the movement region which are sensed one after the other. It also provides a possibility to use the values detected by the pick-up for regulating the size of the glass gobs to be supplied into the parison molds, for example by adjustment of the shears which sever the glass gobs from a stream of molten glass.